1. Field Of The Invention
This invention relates to polypropylene compositions, and more particularly pertains to a novel polypropylene inpact blend having good mechanical properties along with significantly improved optical properties and a method for making same.
2. Description Of The Prior Art
Impact blends of polypropylene modified with minor amounts of certain types of amorphous ethylene-propylene copolymer elastomers and high density polyethylenes are well-known in the art. See, for example, U.S. Pat. No. 3,256,367. As described in U.S. Pat. No. 3,256,367, such modified polypropylene impact blends have improved mechanical properties, particularly improved impact strength, stiffness and heat distortion resistance as compared to unmodified polypropylene or other known polypropylene blends modified with minor amounts of rubber products, such as the above-mentioned ethylene-propylene polymer elastomers, polyisobutylene, and the like.
Furthermore, polypropylene blends comprised of polypropylene and a minor amount of a partially cured monoolefin copolymer rubber, such as an ethylene-propylene polymer elastomer, are known. As described in U.S. Pat. Nos. 3,758,643 and 3,806,558, such blends similarly have improved overall mechanical properties.
Notwithstanding the benefits which have been accomplished by the incorporation of such additives, the polypropylene impact blends referred to suffer from the common disadvantage of having extremely poor optical properties. More specifically, such blends are essentially opaque which severely restricts their use in the manufacture of molded or extruded articles or films.
U.S. Pat. No. 3,515,775 discloses a polypropylene blend of certain types of polypropylene, high density crystalline polyethylene and an amorphous ethylene-propylene copolymer which has relatively high optical properties. However, the types of polymer components employed respectively have relatively low molecular weights and, as disclosed, are useful only in the manufacture of films.
So far as is presently known, no one has previously produced a high impact polypropylene blend having both good mechanical properties, particularly high impact strength, along with acceptable optical properties for use in the manufacture of relatively high clarity molded or extruded articles and/or films.